1. Field of the Invention
The present invention relates to a light receiving circuit, and a photocoupler isolation circuit in which the input portion and the output portion of the circuit require electrical isolation.
2. Description of Related Art
As a method to remove noise components, a method to eliminate components other than signal components by a low-pass filter or a high-pass filter is known widely. However, when a pulsed signal is reproduced by this method, the reproduced signal becomes the sum of odd-number order harmonics of the sinewave component. Therefore, it is necessary to secure frequency bands of 5 times or higher. Thus, when the low-pass filter or the high-pass filter is used, it is necessary to configure so that the pulsed signal can pass through the filter. As a result, many noise components also pass through the low-pass filter or the high-pass filter.
In optical communication devices such as infrared data receiving devices, a relatively low frequency signal (40 kHz) is transmitted to a receiver 102 from a transmitter 101 as an optical signal 103 (see FIG. 8). It is necessary to remove the influence of external disturbing light 104 such as sunlight or illumination light (several kHz or lower) that is a signal having a frequency lower than the frequency of the optical signal 103. Therefore, a differentiating circuit is used to perform the rising edge detection and the removal of noise having frequency components sufficiently lower than the signal frequency. Here, the signal components of high frequency are attenuated when the above-mentioned low-pass filter is used to remove the noise components contained in the pulsed signal. As a result, there are problems that the rising edge gets blunted and the edge detection is impossible.
FIG. 9 shows a block diagram of a receiving circuit device (a pulsed signal demodulation circuit) in the infrared data communications device disclosed in Japanese Unexamined Patent Application Publication No. 11-234098. A photoelectric conversion is performed on an optical signal 103 output from a transmitter 101 (see FIG. 8) and disturbing light 104 by a photodiode 132. The electric current corresponding to the input light level is output to an amplifier 133 from the photo diode 132. By the amplifier 133, the current output from the photodiode 132 is converted into the voltage and the resultant voltage is amplified. An output of the amplifier 133 is input to an amplifier 136 through a coupling capacitor 134 and a pull-up resistor 135.
An output of the amplifier 136 is input to a comparator 137. In the comparator 137, the output of the amplifier 136 is discriminated according to its level by a detection voltage Vth that is defined in advance according to the voltage of a pull-up resistor 135. Then, a square wave pulse, which is the result of the discrimination, is output from a comparator output 138. From the output of the amplifier 133, low frequency components corresponding to the disturbing light 104 are extracted by an auto-bias control circuit (ABCC) 140, and the electric current corresponding to the low frequency components is returned to the input side of the amplifier 133. The auto-bias control circuit 140 includes a primary low-pass filter 141 to extract low frequency signal components corresponding to the disturbing light 104 from the output of the amplifier 133. Moreover, the auto-bias control circuit 140 includes a current-source 142 to convert the output of the low-pass filter 141 from the voltage signal into a current signal and return the resultant current to the input side of amplifier 133.
By employing the above-mentioned constitution, the amplifier 133 amplifies an input pulse signal electric current. Moreover, the electric current corresponding to disturbing light noise components is adjusted by the auto-bias control circuit 140. The electric current output from the auto-bias control circuit 140 returns in the input side of amplifier 133. As a result, the noise components of the disturbing light of the output of the amplifier 133 are removed.
The signal is differentiated once between the input and output of the amplifier 133 by the auto-bias control circuit 140. As for the demodulated pulsed signal, the generation of non-desired vibration is suppressed and the generation of the malfunction pulse is reduced.
For example, a wave pattern output from the amplifier 136 does not have the undesired vibration as shown in FIG. 10B even if incident radiation is influenced by the disturbing light 104 greatly as shown in FIG. 10A. Therefore, as shown in FIG. 10C, the output pulses from the comparator 137 to the comparator output 138 do not produce a malfunction pulse.
Japanese Unexamined Patent Application Publication No. 2003-152649 proposes the following configuration that suppresses continuous wave noise or impulse noise input as disturbing light noise. When an input signal is contained in a signal output from an amplification means, the gain of the amplification means is controlled so that the input signal can have a prescribed signal level. In addition, when noise components are contained in a signal output from the amplification means, the gain of the amplification means is controlled so that the noise component can have a prescribed noise level.
By the way, an isolation circuit, e.g., insulation type amplifier (hereinafter referred to as “isolation amplifier”) has been used to exclude a big common mode noise or to secure safety recently at various places such as factories, plants, and hospitals that use the medical appliances. Particularly big common mode noise often generates power supply difference of 1,000V or larger between high-current electronic devices and high-sensitivity electronic devices in places such as factories or plants and so on. An isolation amplifier using a photocoupler transmits a signal optically and is thereby completely insulated electrically. Therefore, in terms of the noise tolerance, it is superior to an isolation amplifier using magnetism. Recently to enable more precise control for an inverter or an AC servo used in factories or plants, the isolation amplifier is placed, for example, between a motor and a precision instrument such as the microcomputer.